New insights into liver stem cells.

Hepatic progenitor cells are bi-potential stem cells residing in human and animal livers that are able to differentiate towards the hepatocytic and the cholangiocytic lineages. In adult livers, hepatic progenitor cells are quiescent stem cells with a low proliferating rate, representing a reserve compartment that is activated only when the mature epithelial cells of the liver are continuously damaged or inhibited in their replication, or in cases of severe cell loss. Hepatic progenitor cell activation has been described in various acute and chronic liver diseases. Their niche is composed by numerous cells such as Hepatic Stellate Cells, endothelial cells, hepatocytes, cholangiocytes, Kupffer cells, pit cells and inflammatory cells. All these cells, numerous hormones and growth factors could interact and cross-talk with progenitor cells influencing their proliferative and differentiative processes. Hepatic progenitor cells and their niche could represent, in the near future, a target for therapeutic approaches to liver disease based on cell-specific drug delivery systems. Isolation and transplantation of hepatic progenitor cells could represent a new approach for therapy of end-stage chronic liver diseases, as they offer many advantages to transplantation of mature hepatocytes. The possibility of applying stem cell therapy to liver diseases will represent a major goal in this field.